drm/rcar-du: Fix maximum frame buffer pitch computation

The maximum pitch constraint for the hardware is expressed in pixels.
Convert it to bytes to validate frame buffer creation, as frame buffer
pitches are expressed in bytes.

Reported-by: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>